The present invention relates to drag-type drill bits, methods, and systems, and more particularly to replaceable teeth used in such bits.
Background: Rotary Drilling
Oil wells and gas wells are drilled by a process of rotary drilling. In conventional vertical drilling a drill bit is mounted on the end of a drill string (drill pipe plus drill collars), which may be several miles long. At the surface a rotary drive turns the string, including the bit at the bottom of the hole, while drilling fluid (or "mud") is pumped through the string.
When the bit wears out or breaks during drilling, it must be brought up out of the hole. This requires a process called "tripping": a heavy hoist pulls the entire drill string out of the hole, in stages of (for example) about ninety feet at a time. After each stage of lifting, one "stand" of pipe is unscrewed and laid aside for reassembly (while the weight of the drill string is temporarily supported by another mechanism). Since the total weight of the drill string may be hundreds of tons, and the length of the drill string may be tens of thousands of feet, this is not a trivial job. One trip can require tens of hours, and this is a significant expense in the drilling budget. To resume drilling the entire process must be reversed. The bit's durability is very important, to minimize round trips for bit replacement during drilling.
The bit's teeth must crush or cut rock. The necessary forces are supplied by the "weight on bit" (WOB) which presses the bit down into the rock, and by the torque applied at the rotary drive. The WOB and torque are controlled to match the bit type, size, and drilling conditions, but the WOB may in some cases be 100,000 pounds or more. However, the forces actually seen at the drill bit are not constant: the rock being cut may have harder and softer portions (and may break unevenly), and the drill string itself can oscillate in many different modes. Thus the drill bit must be able to operate for long periods under high stresses in a remote environment.
Background: Drag-Type Bits
The simplest type of bit is a "drag" bit, where the entire bit rotates as a single unit. The body of the bit holds fixed teeth, which are typically made of an extremely hard material, such as e.g. tungsten carbide faced with polycrystalline diamond compact (PDC). The body of the bit may be steel, or may be a matrix of a harder material such as tungsten carbide.
As the drillstring is turned, the teeth of the drag bit are pushed through the rock by the combined forces of the weight-on-bit and the torque seen at the bit. (The torque at the bit will be somewhat less than the rotary torque, due to drag along the length of the drill string. The torque at the bit may also contain a dynamic component due to oscillation modes of the drill string). Since the weight-on-bit and the rotary torque are controlled by the driller, the net thrust vector seen at the tooth face will be slightly uncertain; but the normal range of torque and WOB values will imply only a relatively small range of angular uncertainty for each tooth's net force vector. (The rate-of-penetration and the hardness of the formation also have some effect on the orientation of the thrust vector seen at the tooth.) Thus each tooth can be aligned to an expected thrust direction, within a cone of a few degrees of uncertainty.
Background: Failure Modes of PDC-Type Teeth
The drilling environment is a harsh one, with high shock loading, high temperatures, and abrasive fluid flows. Even with modern superhard materials (such as PDC facings on a tungsten carbide body), drilling contractors often must perform expensive "trips" merely to replace drill bits.
All drill bit teeth can be expected to fail eventually. However, an important question is: How do they fail? PDC-type drill bit teeth have at least three important failure modes, as illustrated schematically in FIGS. 20A-20C. (These failure modes are illustrated for the bullet-type tooth of FIG. 20, but are relevant to many other tooth types as well.)
The most innocuous mode, illustrated in FIG. 20A, is inward abrasive wear of the cutting face. The side of the tooth's superhard face 2040 is gradually eroded inward, so that portion 2000' of the tooth's volume is gradually removed.
A less welcome failure mode, illustrated in FIG. 20B, is fracture. The force on the tooth's face is not distributed evenly, so it is possible for failure in shear to occur (where part of the face, and the part of the body behind it, breaks away from the rest of the tooth). This is a particularly damaging failure mode, since the separated tooth fragment 2000" is likely to be encountered by the next tooth behind it. The separated tooth fragment 2000", unlike the rock being drilled, is just as hard and has just as high a yield stress as the tooth behind it. Thus the separated tooth fragment 2000" has some chance of breaking the following tooth also. There is thus some chance of a "chain reaction," where trash from one broken tooth causes tooth breakage to propagate to corresponding locations all the way around the bit.
An even more unwelcome failure mode, illustrated in FIG. 20C, is "prying out" failure, where all or most of a single tooth's volume is removed from its socket. The single mass of tooth material has an even better chance of damaging the following tooth.
Background: Angled Teeth
Some attempts have been made to use angled teeth in drag bits. FIG. 18 shows a conventional drill bit tooth 1810 which contains two nonparallel axes. This design has not come into wide use. The cantilevered front portion 1820 of the tooth provides a weak spot where large fragments or stray trash can exert outward forces; such outward forces can cause the inside of the bend to begin to fail in tension, and cracks can then propagate quickly. Even without trash or cuttings wedging under the front portion of the tooth, transient impacts at the face of the tooth can also translate to a net outward torque at the bend of the tooth, and this can lead to rapid failure. Thus such teeth are susceptible to failure modes which include being levered up, or failing in tension at the inner radius of the angle, or failing in shear across the shank.
FIG. 19 shows a different conventional drill bit tooth which has less of its length protruding from the body. This bit contains a face 1920 bent at an angle of roughly 90 degrees to the shank 1910 of the tooth. Here the very sharp angle between the cutting face and the shank produces a point of stress concentration, which is conducive to possible failure. Moreover, the thickness of material through which a shear-failure crack or defect must propagate through is at most the thickness of the tooth's shank 1910. Such teeth are often backed by a portion of the steel bit body, but still the failure resistance is less than optimal. When the tooth starts to fail, its resultant cutting radius changes rapidly.
Background: "Bullet"-Type Teeth
FIG. 20 shows a sectional view of bullet-type drill bit tooth 2000 as disclosed in a sample embodiment of commonly-owned U.S. Pat. No. 5,558,170 to Thigpen et al. This patent, which is hereby incorporated by reference, describes (among other teachings) a drag-type drill bit in which the teeth are cylindrical, with a hemispherical back end 2050 for seating into a milled pocket. Typically the body 2010 of the tooth is a hard strong material, such as cemented tungsten carbide, and its front end is typically a flat circle which is coated with a superhard material 2040 such as a polycrystalline diamond compact ("PDC"). By using a spherical mill, an open cylindrical pocket with a spherically-shaped end surface can be machined into a steel bit body 2060 to provide a reasonably close fit to such a tooth, and the tooth can be brazed into the pocket to form a high-strength joint. By designing the pocket so that its sidewalls extend up to partially enclose the top of the tooth 2000, some resistance against prying-out of the tooth is obtained.
This configuration provided an improvement over some of the shortcomings of conventional PDC-type bits and teeth. The tooth's main axis is nearly parallel to the main force vector seen during cutting, so that shear failure is well opposed. Moreover, the stiffness of cemented carbide materials is higher than that of steel, so the rigidity of this mounting helps to suppress chatter and analogous instabilities. A further advantage of this configuration is that the body of the tooth, behind the superhard face, provides an additional hard sliding surface area for rock contact when abrasive wear begins to reduce the area of the tooth face.
Drag-Bit Drilling with Multi-Axial Tooth Inserts
The present application discloses drill bits, teeth, and manufacturing and replacement methods for drag-type drill bits with inserted teeth. The innovative teeth are bent: each includes a front portion which is bonded to the body of the bit along a substantial part of its length, and a shank portion which is not parallel to the front portion. (Preferably the front portion is supported along a length which is greater than half the maximum diameter of the shank.) The shank portion provides more secure attachment than is obtained from a conventional "bullet"-type tooth.
The disclosed innovations, in various embodiments, provide one or more of at least the following advantages:
Stronger bonded assembly; PA1 Compatibility of different tooth geometries with secure mounting to a given bit design; PA1 More resistance to "pry-out" failure; and PA1 Field replacement: when a tooth breaks, the pocket is usually not destroyed. Thus a repair technician can actually replace a damaged tooth on the drill rig floor. This provides additional flexibility to make field repairs, and reduces the need to stock and rapidly transport drill bits.